Analysis of a shred of 68-million-year-old Tyrannosaurus rex protein - along with that of 21 modern species and an extinct mastodon - confirms that dinosaurs share common ancestry with birds, particularly chickens and ostriches, and to a lesser extent, alligators.

Tyrannosaurus rex, the "tyrant lizard", has confirmed its place on the vertebrate (backboned animal) family tree, thanks to the new study, which was made possible thanks to an extremely unusual T rex fossil found in 2003.

Even though most fossils contain only preserved hard tissues, like bones or shells, this one contained some of the dinosaur's soft tissues, such as collagen and blood vessels.

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Now, Dr Chris Organ of Harvard University and the Museum of Comparative Zoology in Cambridge, Massachusetts and his colleagues have analysed the proteins in the collagen preserved from that T rex fossil and another fossil, a mastodon that was some 160,000- to 600,000-year-old.

They compared these proteins with proteins from 21 modern-day vertebrates, including chickens, ostriches and elephants.

The proteins from T rex were most similar to those of birds, while those from the mastodon were most similar to elephants, they conclude in the journal Science.

"These results match predictions made from skeletal anatomy, providing the first molecular evidence for the evolutionary relationships of a non-avian dinosaur," says Dr Organ

Coauthor Prof John Asara a of Beth Israel Deaconess Medical Centre and Harvard Medical School, adds: "We determined that T rex, in fact, grouped with birds - ostrich and chicken - better than any other organism that we studied. We also show that it groups better with birds than modern reptiles, such as alligators and green anole lizards."

While scientists have long suspected that birds are dinosaurs' closest living relatives, for years that hypothesis rested largely on similarities in bird and dinosaur skeletons.

These new results strengthen what researchers had predicted from looking at whole fossils from these animals: that dinosaurs share more of their genetic makeup with birds than with other modern-day vertebrates, and that extinct mastodons and modern-day elephants are also closely related.